docs: def _get_phi_al_regional(self, C, mag, vs30measured, rrup):
          """
          Returns intra-event (Tau) standard deviation (equation 26, page 1046)
          """
          phi_al = np.ones((len(vs30measured)))
  
          idx = rrup < 30
          phi_al[idx] *= C['s5']
  
          idx = ((rrup <= 80) & (rrup >= 30.))
          phi_al[idx] *= C['s5'] + (C['s6'] - C['s5']) / 50. * (rrup[idx] - 30.)
  
          idx = rrup > 80
          phi_al[idx] *= C['s6']
  
          return phi_al